1. The Field of the Invention
The present invention relates to systems, methods, and apparatus for heating metal products. More specifically, the present invention relates to oscillating induction furnaces and methods for heating metal products in an oscillating induction furnace.
2. The Relevant Technology
The production of steel and other metal products requires that the metal products be maintained at a certain temperature subsequent to the initial casting and refinement of the metal products from raw materials. As an example, iron is typically molten and cast into steel slabs in an initial procedure in the steel making process. These steel slabs must be later rolled or otherwise shaped into specified dimensions. In the interim between casting and rolling, however, the steel slabs cool off to a temperature below the optimum rolling temperature. To bring the steel slabs back up to the optimum rolling temperature, the steel slabs are heated in a furnace.
Several types of furnaces have been used for heating metal products. One type of furnace frequently used for heating and reheating metal products is the gas fire furnace. The gas fire furnace is, in its simplest form, a large gas oven in which the metal products are placed. The gas fire furnace is capable of heating metal products to a broad range of temperatures and is relatively cost efficient to operate. Nevertheless, the gas fire furnace has drawbacks in certain applications. For instance, the gas fire furnace generally has a low throughput, is expensive to construct, and occupies a large amount of space in the mill. Additionally, it is not always easy to predict the exact time when the rolling equipment or other downstream processing station will be ready to process the reheated metal product. Accordingly, gas fire furnaces require a holding area in which to maintain the metal products at a target temperature until they are needed. The holding area requires additional expense to construct and operate, consumes additional space in the steel mill, and uses additional energy to operate.
Consequently, in applications where capital and space are limited and where a high throughput is required, the prior art has looked to less expensive, more compact furnaces. One such type of furnace is known as the induction furnace. The induction furnace typically comprises a large inductor coil to which is applied an alternating current of great magnitude and through which the metal product is passed. The induction furnace operates on the principle of resistive heating. That is, when a metal product is passed through the induction furnace, the inductor coil causes magnetic flux of varying magnitude and direction to pass through the metal product. The changing magnetic flux induces current in the metal product which encounters internal electrical resistance. The current, in overcoming the internal electrical resistance, generates heat according to the equation: P=I.sup.2 R, where I is the amount of current induced within the metal product and R is the internal electrical resistance of the metal product. The variable P represents the power expended and is proportional to the amount of heat generated within the metal product.
Induction furnaces also have their limitations, one of which is that different segments of the metal products are often heated at differing rates and thus attain divergent temperatures. The differing rates of heating are attributable to the configuration of the induction furnace in which the separate windings of the induction coil are typically spaced several inches apart from each other. Also, unless a very long inductor coil or series of inductor coils is used, the metal product must be left within the inductor coil for an extended period of time. Thus, portions of the metal products which are in closer proximity to the individual windings of the inductor coils receive greater amounts of magnetic flux than those portions in lesser proximity thereto. Consequently, a correspondingly greater current is induced within the portions in closer proximity to the windings, and these portions therefore attain a higher temperature than the portions in lesser proximity to the windings, resulting in adjacent segments of the metal product being nonuniformly heated to temperatures that vary greatly. This nonuniform heating of adjacent segments is known as temperature striping.
To rectify temperature striping, the prior art has attempted to oscillate the metal product back and forth within a vertically oriented inductor coil. Oscillation of the metal product also raises problems, however. For instance, when oscillating the metal product, the metal product is typically placed on a hydraulic ram which raises the metal product up and down within the inductor coil. The metal product must be moved onto and off of the hydraulic ram, which consumes processing time and reduces throughput. Also, the sizes of the metal products which can be raised on hydraulic rams are also limited, typically to under about 10 tons per slab in prior art induction furnaces. Furthermore, only a single metal product can be heated in such an induction furnace at a single time, and provision must be made for maintaining a target temperature of the heated metal slabs within the induction furnace once the heated metal slabs reach the target temperature and until the mill is ready for the heated metal slabs. Thus, a separate holding area is also generally required, causing the drawbacks discussed above in regards to a holding area.
As an additional limitation of prior art oscillating induction furnaces, typically only a single metal product can be passed through a prior art oscillating induction furnace at any one time. Throughput of the prior art induction furnaces is therefore limited.
Accordingly, a need exists in the art for an induction furnace which overcomes the above-discussed problems. Specifically, an induction furnace is needed which does not incur significant temperature striping, which occupies minimal space within a steel mill, and which has a high throughput. Such an induction furnace is also needed which can maintain the metal products at a target temperature until the metal products are needed, which can heat metal products of great weight, and which can heat combinations of metal products concurrently.